Method for multisensor arrangements

ABSTRACT

The present invention is providing a method and a sensor device arrangement for a scanning digital radiographic system offering a multiple sensor arrangement producing continuous image data acquisition along the entire length of the detector device arrangement. By cutting the sensor die short edges along a die cut (46) with a predefined angle in relation to the scanning direction there is no longer a total loss of data at any position along the longitudinal extension of the detector arrangement. Instead some minor loss pixels (40) is encountered over a certain range of the scanning width coinciding with the joint of two adjacent sensor dies, in which range the image can easily be fully reconstructed by means of the electronic circuitry.

TECHNICAL FIELD

The present invention relates to sensors for x-ray imaging and moreparticularly to a multi sensor arrangement for a scanning digitalradiographic system.

PRIOR ART

Since the middle of 1950 a common method to produce overview x-rayimages of teeth and jaws for use in dentistry is the so called panoramicx-ray method. FIG. 1. demonstrates a typical prior art set up. Assumingthe head of the patient is oriented in an upright position, the methoduses a movable x-ray source 1 with the, beam collimated to a narrow (3-6mm) width in the horizontal dimension, and elongated at least 12 cm inthe vertical dimension. The collimation is done with a narrow slit in apiece of sheet metal made of an x-ray absorbing material placed at asuitable distance from the focal spot of the x-ray tube, but before theradiation reaches the patient. On the opposing side of the patient,relative to the x-ray source, a second metal plate 3 with a narrowsecond slit, corresponding to the fan shaped beam is placed. The slitsand the x-ray source with it's collimator are rigidly attached to eachother. A film 5 is placed further away from the x-ray source, in thedirection of the x-ray beam, at a position behind the second slit.During the exposure, usually taking 15-20 seconds for an ordinarypanoramic x-ray, 30 cm wide, the arrangement of x-ray source and slit isrotated in a direction 3 around the head of the patient in a controlledmanner so that a rotational center 7 of the imaging system will besituated within the head of the patient. At the same time, the imagingsystem will move relative to the film which will be exposed by radiationthrough the patients head and the second slit, piece by piece, until allof the film has been exposed during the time the imaging system wasrotated around its rotational center 7.

By controlling the film speed relative to the object, the x-ray beam andthe projection geometry with the rotational center assumed as a "virtualfocus" that is moved during the exposure, it can be demonstrated thatonly a predetermined layer in the object is sharply depicted. In orderto bring this sharp layer to coincide with the dental arches and tocompensate for magnification variations in the vertical and horizontaldimensions, a rather complex motion sequence is required, where thevertically oriented rotational axis 7 is not fixed within the patientshead but will be moving along a continuous path in the horizontal planeapproximately parallel to the occlusal plane according to FIG. 2.

A good reference for a popular technical description of conventionalpanoramic radiography is found in chapter 2, "Theory of RotationalPanoramic Radiography", second edition of "Panoramic radiology" by OlafLangland, Robert Langlais, Doss McDavid, Angelo DelBalso, printed by Lea& Febiger, Philadelphia 1989.

The traditional method can be viewed, as indicated in FIGS. 3 and 4, asa method to add an indefinite number of images, each of them withindefinitely low exposure, in size corresponding to the slit, directlyon to the analog film medium, simultaneously moving the film past theslit as described above. However, the method would work equally wellusing a large but finite number of discrete steps both in the x-raysource/slit assembly motion and in the motion of the film.

It is obvious that the described method could be modified to utilize forinstance semiconductor detectors replacing the film. Three examples ofrelevant patents are the U. S. Pat. Nos. 4,878,234, 4,823,369 and5,018,177.

The first patent document U.S. Pat. No. 4,878,234 deals with a methodand an apparatus to use one or several CCD (Charge Coupled Device)detectors for both detecting the image information and for simulation ofthe film motion directly in the imaging area by, since the earlydevelopment of CCD:s, the well known method named TDI (Time DelayedIntegration) wherein the individual pixel charges can be moved tosimulate film motion by the clocking sequence of the detector. Thismethod has the same limitations as the film method regarding thenecessary complex motion pattern to adjust the horizontal magnificationand to determine the position of the sharp layer. An obvious drawback isthat the position of this layer has to be pre-determined, before theexposure. The image is digitized during the exposure sequence andimmediately subsequently presented on a video monitor.

The second patent publication U.S. Pat. No. 4,823,369 deals with amethod and an apparatus to capture a large number of overlapping imagesusing a semiconductor image detector with a dimension corresponding tothe above described slit, in real time postprocessing each of the imagesby adding pixels adjacent to each other to create fewer columns of imagedata to reduce the amount of data. Each of the resulting conbinedcolumns would correspond to a readout register of a detector asdescribed in the previous U.S. Pat. No. 4,878,234 or to an ordinaryimage column with reduced spatial resolution. Image data from the set ofcolumns after postprocessing is stored at separate locations in a largeimage memory. A reconstruction of panoramic images is performed afterthe completion of the exposure by adding the images stored in memory toanother memory, slightly positionally shifted relative to each other,until one or more resulting images with a size corresponding toapproximately 12.5×30 cm has been totally covered using the sums of alarge number of smaller images.

This method reduces the spatial resolution or restricts the subsequentreconstruction to a restricted number of predetermined layers dependingon how many groups of image column data that will be created during thedata reduction.

It should be noted that in both above disclosed patent documents asecondary diaphragm is included as a necessary requirement in all theindependent claims. However, a secondary diaphragm would be completelyunnecessary when a large area film is replaced by a detector which onlypicks up radiation falling within the borders of the sensitive area.

The third document U.S. Pat. No. 5,018,177 discloses a method and anapparatus for producing among other possible projections, panoramicradiograms, using one single, vertically oriented line detector,consisting of a number of pixel detectors. This document mainly dealswith the idea to divide the complete predetermined scanning period intoa series of time intervals which are a function of the elapsed timewithin the total scanning period. The signal created by radiation in thedetector is integrated during each time interval. At the end of eachtime interval the analog signal from the detector is converted todigital data. By means of data processing a complete two dimensionalradiogram is produced from the set of column data from the detector. Thedisclosure also deals with a way to calculate the described timeintervals from a given projection geometry and a given predeterminedscanning time interval to obtain a panoramic radiogram.

Thus a common method for imaging large objects is to scan over theobject using a linear sensor, for example a diode array, or a narrowtwo-dimensional sensor, for example a CCD or any other two dimensionalpixel array. Since the length of the sensor is limited by the wafer sizeand the processing capabilities to about 5-8 cm the scan width using asingle sensor is also limited.

The solution for wider objects is to either scan in two dimensions or toincrease the scan width by mounting a number of sensors in line.Two-dimensional scanning complicates the mechanics and also tends totake too much time for many applications.

The general available sensors today are rectangular or square formed.When mounting multiple rectangular or square sensors in line data willbe always lost in the border region between the individual sensors. Theactive area can not be extended all the way to the edge of the die sinceroom is needed for electrical connection buses and the properties of thesemiconductor close to the cutting edge are not ideal (FIG. 7). Somespace is also needed between the dies for alignment of the pixels. Theloss of sensitive width is generally around 100-200 μm.

Therefore there is a need for a method and a device which facilitate theforming of a long rectangular sensor consisting of multiple sensors inline with a minimum loss of data at the borders between the sensors.

SHORT DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method and a sensordevice for a scanning digital radiographic system which offers amultiple sensor arrangement producing continuous data acquisition alongthe entire length of the sensor device designed according to theinvention.

By placing the sensor die border edges at an angle in relation to thescanning direction there is no longer a total loss of data at anyposition along the longitudinal extension of a long rectangulardetector. Instead some loss of data is encountered over a certain rangeof the scanning width. If the portion of data that is lost is smallcompared to the total amount of data collected from that position theimage can easily be reconstructed by means of the imaging scanningsystem circuitry or software. The invention as claimed is set forth bythe independent claim 1 and in the dependent claims 2-6 a number ofdifferent embodiments according to the invention are set forth.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be described by preferred embodiments to becontemplated with reference to the accompanying drawings wherein likereference numerals are used throughout to designate like parts. In thedrawings:

FIG. 1 shows a traditional panoramic narrow slit rotational radiographywith film;

FIG. 2 shows dental arch with a motion path of the rotational axisduring an exposure sequence;

FIG. 3 shows a motion of the detector during a part of an exposuresequence from one position to another and at the same time the x-rayfocal spot moves along the path indicated by the arrow and therotational axis is moved along a path according to FIG. 2;

FIG. 4 shows an adding up of vertically oriented, horizontally shifted,narrow x-ray images creating an added up radiogram covering a largerarea than a single image;

FIG. 5 demonstrates in a plane view a preferred embodiment utilizing adetector comprising a sensor arrangement according to the presentinvention;

FIG. 6 demonstrates different arrangement for a scanning system with ascanning height larger than the single sensor segment length, thedesired imaging window being indicated with a dotted line for eachsensor arrangement;

FIG. 7 shows a close up of the sensor segment joint of FIG. 6Ademonstrating a detailed picture of the border between the two sensorsegments with rectangular shape mounted in line;

FIG. 8 shows a close up of FIG. 6B as a detailed picture of the borderbetween sensor segments using rectangular sensors mounted in an angle tothe scanning direction; and

FIG. 9 shows details of the border between sensor segments according tothe present invention demonstrated in FIGS. 6C-6E having pixels orientedin the scanning direction whereby only a fraction of the data will belost at the border.

DESCRIPTION OF A PREFERRED EMBODIMENT

An illustrative embodiment of the current invention comprises accordingto FIG. 5 a case 15 containing an x-ray source 13 and in rigidmechanical connection with a rectangular imaging detector 19 having itslong sides vertically oriented. This x-ray source/detector assembly 18is rotatable around a vertically oriented axis 17 that is movablebetween any positionswithin a horizontal plane 16. Close to the x-raysource 13 in the case 15 there is a collimator 14 provided with a slitwhich together serves to direct the x-ray beam to the detector and toadapt the cross section of beam at the detector to approximately matchthe active area of the detector 19 comprising a sensor arrangementaccording to the present invention. The beam area is matched to thedetector area to avoid more radiation to the patient than what isrequired for the acquirement of the images. A second reason for limitingthe beam area is to reduce the amountof secondary radiation induced inthe object. Excess secondary radiation decreases the image quality.

In FIG. 6 is demonstrated a number of different embodiments to obtain anelongated detector area which is larger than what may be obtained by asingle semiconductor die for a scanning dental digital radiographicsystemaccording to FIG. 5.

FIG. 6A shows a method according to the state of the art to join two ormore sensor elements. In FIG. 7 is demonstrated an enlarged view of thearea around the border where the adjacent sensor elements 30 are joinedtoform the total elongated rectangular detection area wanted. This willby necessity result in a data gap in the image at the joining positionof thetwo elements 30 in the embodiment of FIG. 6A, among other thingsdue to thesurface occupied by the metal bus 42 as well as the die cut 46itself.

A basic arrangement utilizing an idea along with the general method ofthe present invention is demonstrated in FIG. 6B, utilizing a number ofsmaller sensor elements 32 formed into an array by tilting theindividual sensor element to obtain an overlapping detecting area in thescanning direction. This will however lead to a waste of active area asindicated by the ideal scanning,window inserted in FIG. 6. Theorientation of the rows and columns of the sensor pixels 40, asdemonstrated in FIG. 8, also complicates the final reconstruction of theimage. Anyhow 6B demonstrates a practical cheap way of arranging anelongated sensor by means of a number of easy available small elements32. Compared to the embodiments according to FIGS. 6C to 6E, there isquite some waste of detector surface, but because of the availability ofsmall rectangular or square elements it will still be economicallyinteresting. By means of the slanting border between the elements therewill be no points along the vertical direction lacking data pointsduring the scan. At the borders between individual elements there willbe only a small fraction of data missing which easily may bereconstructed by means of hardware or software. The actively operatingrectangular surface indicated by the dotted line may easily be definedby a suitable mask.

According to FIGS. 6C to 6E, to save semiconductor surface, theindividual sensor elements are not cut using right angles at one orboth, generally, of the short ends as in FIGS. 6A or 6B, but insteadthey are cut with a slanted end side as is demonstrated in the examplesof FIGS. 6C, 6D and 6E, then a number of sensors may be mounted in linestill with only a fractional loss of data at the border region betweenthe individual adjacent sensor elements. This is illustrated in theembodied applicationsof FIGS. 6C-6E demonstrating, according to thepresent invention, individually shaped sensor elements 34, 36, and 38.

FIG. 6C illustrates a case when the entire active detector area islocated inside the imaging window and the rows of the sensor pixels 40are oriented in the scanning direction as may be seen in the enlargedview of FIG. 9. FIG. 6D demonstrates a case using symmetrical sensorsections 36, where the die slanted short end cuts are parallel, whileFIG. 6E illustrates a case having the die slanted short end cutsanti-parallel, i.e. at a straight angle i relation to each other for thetwo short ends of each separate sensor die. However in the embodimentsof FIGS. 6D and 6Ea little fraction of active area will be wasted at theuppermost, and lowermost portions of the final sensor arrangement as acontribution to the geometry of the die which allows for the samemanufacturing process for the short end edges of all the die elements.

The loss of data is controlled by the pixel size, the distance betweenactive areas, the sensor width and the cutting angle. For example asensorwidth of 10 mm, a pixel size of 100 μm, a distance between activeareas of 200 μm and a cutting angle of 45° gives a loss of data of 4%along the border. An image with such a small loss of data can easily bereconstructed. The minor loss of pixels encountered over the certainrangeof the scanning width coinciding with the joining of two adjacentsensor dies the image may in this range easily be fully reconstructed bymeans ofthe electronic circuitry or the software of the panoramicsystem.

In the figures and in the example above a cutting angle of 45° has beenused. The cutting angle may however vary in a wide range. Theoptimalangle depends on sensor dimensions, pixel size and distance fromlast pixelto the edge of the die. The slanted cut sides of the die areprovided with a corresponding metal bus 42 or readout section 44according to standard semiconductor technique well known to a personskilled in the art.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparturing from the spirit and scope thereof, which is defined by theappended claims.

I claim:
 1. A method for extending the physical length of a rectangularx-ray detector for a scanning digital radiographic system devicecomprising a number of individual sensor element dies, characterized byarranging the border line between two adjacent individual sensorelements (34, 36, 38) with a predefined angle in relation to thescanning direction of said radiographic system thereby in the scanningdirection creating an overlapping detector sensor element area enablinga continuous data acquisition along the entire length of the multiplesensor element arrangement.
 2. The method according to claim 1,characterized by cutting at least one short end of a detector sensor die(34) to a predefined angle in relation to the scanning direction wherebywhen arranging two such detector dies into an elongated detectorarrangement rows and columns of the sensor die pixels (40) areorientated parallel and perpendicular, respectively to the scanningdirection,
 3. The method according to claim 1, characterized by cuttingthe short ends of a detector sensor die (36) in parallel to a predefinedangle in relation to the scanning direction whereby when arranging twoor more such detector dies into an elongated detector arrangement therows and columns of the sensor die pixels (40) are orientated paralleland perpendicular, respectively to the scanning direction.
 4. The methodaccording to claim 1, characterized by cutting the short ends of adetector sensor die (38) anti-parallel to a predefined angle in relationto the scanning direction whereby when arranging two or more suchdetector dies into an extended detector arrangement the rows and columnsof the sensor die pixels (40) are orientated parallel and perpendicular,respectively to the scanning direction.
 5. The method according to claim1, characterized by arranging a number of standard rectangular or squaredetector elements (32) in an elongated row in which the individualelements are positioned with a slanted angle in respect to the scanningdirection.
 6. The method according to claim 5, characterized byarranging a mask on top the arrangement of detector elements (32) todefine an elongated active sensor area in a direction perpendicular tothe scanning direction.
 7. An image sensor arrangement utilizing themethod of claim 1 characterized in that said sensor arrangement is builtup by any convenient combination of individual detector sensor dies (32,34, 36, 38) forming an elongated rectangular x-ray detector for ascanning digital radiographic system, whereby such a detecterarrangement by means of the overlapping detector sensor element areas inthe scanning direction enables a continuous image data acquisition alongthe entire length of the multiple sensor arrangement with no imaginggaps.
 8. An image sensor arrangement utilizing the method of claim 2,characterized in that said sensor arrangement is built up by anyconvenient combination of individual detector sensor dies (32, 34, 36,38) forming an elongated rectangular x-ray detector for a scanningdigital radiographic system, whereby such a detector arrangement bymeans of the overlapping detector sensor element areas in the scanningdirection enables a continuous image data acquisition along the entirelength of the multiple sensor arrangement with no imaging gaps.
 9. Animage sensor arrangement utilizing the method of claim 3, characterizedin that said sensor arrangement is built up by any convenientcombination of individual detector sensor dies (32, 34, 36, 38) formingan elongated rectangular x-ray detector for a scanning digitalradiographic system, whereby such a detector arrangement by means of theoverlapping detector sensor element areas in the scanning directionenables a continuous image data acquisition along the entire length ofthe multiple sensor arrangement with no imaging gaps.
 10. An imagesensor arrangement utilizing the method of claim 4, characterized inthat said sensor arrangement is built up by any convenient combinationof individual detector sensor dies (32, 34, 36, 38) forming an elongatedrectangular x-ray detector for a scanning digital radiographic system,whereby such a detector arrangement by means of the overlapping detectorsensor element areas in the scanning direction enables a continuousimage data acquisition along the entire length of the multiple sensorarrangement with no imaging gaps.
 11. An image sensor arrangementutilizing the method of claim 5, characterized in that said sensorarrangement is built up by any convenient combination of individualdetector sensor dies (32, 34, 36, 38) forming an elongated rectangularx-ray detector for a scanning digital radiographic system, whereby sucha detector arrangement by means of the overlapping detector sensorelement areas in the scanning direction enables a continuous image dataacquisition along the entire length of the multiple sensor arrangementwith no imaging gaps.
 12. An image sensor arrangement utilizing themethod of claim 6, characterized in that said sensor arrangement isbuilt up by any convenient combination of individual detector sensordies (32, 34, 36, 38) forming an elongated rectangular x-ray detectorfor a scanning digital radiographic system, whereby such a detectorarrangement by means of the overlapping detector sensor element areas inthe scanning direction enables a continuous image data acquisition alongthe entire length of the multiple sensor arrangement with no imaginggaps.